One type of wellhead assembly, particularly used offshore, is known as a horizontal tree. The well has a wellhead housing which contains casing hangers, each secured to a string of production casing that extends into the well. The tree mounts on top of the wellhead housing. The tree has a vertical bore and a horizontal or lateral production flow outlet. A tubing hanger lands in the bore of the tree and is secured to a string of production tubing extending through the casing hangers and into the well. The tubing hanger has a lateral flow passage that registers with the lateral passage of the horizontal tree.
A plug, normally wireline retrievable, fits in the vertical passage of the tubing hanger above the horizontal passage. A tree cap fits above the tubing hanger in the bore of the tree. The tree cap may have a vertical passage within which the retrievable plug fits. A corrosion cap fits over the upper end of the tree.
A tubing annulus between the tubing and the casing communicates to a lower annulus port formed in the tree. This port leads through an annulus passage to an upper annulus port which extends into the bore of the tree above the tubing hanger seals. One or more valves are used to open and close the tubing annulus. The upper tubing annulus port communicates with a void that is located between the tubing hanger wireline plug and the seal of the internal tree cap.